Before I implement it properly, is this the kind of api that's desired?

"""
import os
import io

class MyOpener:
    def __init__(self, dirname):
        self.dirfd = os.open(dirname, os.O_RDONLY)

    def open(self, path, flags, mode):
        return os.openat(self.dirfd, path, flags, mode)

myop = MyOpener("/tmp")
f = open("testfile", "w", opener=myop.open)
f.write("hello")
"""